The present invention relates to a printed circuit board (PCB) for transferring differential signals and, more particularly, to a printed circuit board in which matching of differential characteristic impedance is maintained.
As electronic information devices typified by computers have been operated at high rates in recent years, there arises a need for transferring higher-frequency digital signals between LSIs and within printed circuit boards. A currently widely accepted method of satisfying this requirement consists of preparing a pair of signal lines, passing signals of opposite polarities through the two lines, and recognizing a signal from the resulting potential difference. This is known as a differential signal transmission method.
In the differential signal transmission method, signals of opposite polarities are transmitted through a pair of signal lines. Therefore, if the two lines are different in electrical length (trace lengths represented based on delay time), a skew (phase deviation of signal waveform) is produced by a difference in propagation time between the differential signal pair at a receiving end even if they are opposite in polarity at a transmitting end. This generates unipolar noises. Therefore, in the differential signal transmission method, meander trace configurations are generally used. That is, one signal line of shorter electrical length out of the two signals providing a differential signal is meandered to match the two signals in terms of electrical length.
An example in which a transmission line having a meander trace configuration is mounted on a printed circuit board 2200 is shown in FIG. 22. In an interval 2202 indicating a corner shown in the figure, the outer trace is longer than the inner trace, thus producing a skew. In the present example of mounting, the skew is adjusted by causing the traces within an interval 2201 to meander.
The meander trace configuration is shown more particularly in FIG. 20. An interval 2008 represents a meander trace of a skew-adjusting portion, while indicated by 2009 and 2010 are non-skew-adjusting portions not involved in the skew adjustment. The length of a conductive trace subjected to the skew adjustment from a transmitting end 2000 to a receiving end 2002 is longer than the length of a conductive trace not subjected to a skew adjustment from a transmitting end 2001 to a receiving end 2003. The skew is thus adjusted by stretching one trace of the differential trace pair in this way.
With this conventional method of skew adjustment, however, a convex transmission line segment 2004 providing a wider trace pair spacing 2007 than in the non-skew-adjusting portions is spliced to a transmission line segment 2005 having a same trace pair spacing 2006 as in the non-skew-adjusting portions. These convex and non-convex transmission line segments are arranged at regular intervals. Consequently, there arises a problem that the differential characteristic impedances are mismatched.
FIG. 21 is a graph showing the results of an analysis of the reflectivity of a pulsed wave having a rise time (Tr) of 100 psec when it entered a differential trace pair of the conventional meander trace configuration of FIG. 20. The analysis was performed using a circuit simulator. In the present simulation, symmetric strip traces made of FR-4 substrate material that is currently widely used were assumed. The distance between a signal layer and a grounding layer was 0.142 mm. The differential trace pair spacing 2006 in each non-skew-adjusting portion was 0.167 mm. The differential trace pair spacing 2007 of the convex transmission line segment 2004 was 0.281 mm. The trace length of the convex transmission line segment 2004 was 0.5 mm. The trace length of each of the non-skew-adjusting portions 2009 and 2010 was 100 mm. In the illustrated example, 5, 10, 20 convex transmission line segments 2004 were formed in succession. As can be seen from the graph, when the skew has been adjusted, reflection was produced at the convex transmission line segments 2004. The reflectivity increased as 0.6%, 1.1%, and 1.2% when the number of the transmission line segments increased as 5, 10, and 20, respectively. That is, the reflectivity was increased with increasing the number of the convex transmission line segments. In this way, the skew adjustment of the conventional meander trace configuration is affected by the reflections due to the mismatch of the differential characteristic impedances. The effect becomes larger with increasing length of the skew-adjusting portion.
Furthermore, in the skew adjustment of the conventional meander trace configuration, the amount of skew adjustment per unit length of the skew-adjusting trace is small, and therefore, it is necessary to secure a large interconnect area for the skew adjustment.
As another well-known technique, JP-A-2008-244703 describes a method of forming vias having different shapes in differential trace pairs extending across a plurality of signal layers. JP-A-2008-153386 provides a method of adjusting skew by mounting capacitive stubs on conductive portions. In any of these methods, the mismatching of the differential characteristic impedances at electrical length-adjusting portions is unavoidable.
The effect of the mismatching of the differential characteristic impedances on the signal quality becomes more conspicuous with increasing rates at which signals are transmitted. Currently, there is a demand for a technique capable of solving both of the problems of the skew adjustment and the mismatching of the differential characteristic impedances.